The Effects of Alloying Elements on the Corrosion of Rebar Steel in a Chloride Environment

Abstract

The corrosion behaviors in chloride environment of two commercial low-alloy steel bars were studied. Through cyclic wetting tests, accelerated corrosion experiments ranging from 1 to 576 h were conducted on low-alloy bars and original bars. Techniques such as OM, SEM, EDS, AFM, and XRD were employed to characterize the corrosion emergence and expansion behaviors of these bars in a simulated marine wetting and sun exposure environment. The designed low-alloy corrosion-resistant rebar achieved a 500 MPa yield strength. In each corrosion cycle, its corrosion loss and rate were lower than those of same-strength ordinary rebars. Analysis of the rust layer’s macro and micro morphology and alloy element distribution revealed alloy elements had little effect at corrosion initiation. In later corrosion, their enrichment led to a denser rust layer, effectively blocking corrosion expansion and chloride salt infiltration. After 72 h of accelerated corrosion, the corrosion rate growth of both bars slowed. The inner rust layer’s electrochemical potential increased, and local corrosion pits turned into uniform corrosion. The inner rust layer of the rebar formed more stable chromic acid with ionic compounds, reducing corrosion sensitivity. This study offers insights into steel bar corrosion and alloy element roles, guiding the preparation of low-alloy corrosion-resistant steel bars.

1. Introduction

Steel reinforcement corrosion is a global issue affecting construction engineering, with chloride-induced corrosion being a leading cause of premature structural failure in coastal countries such as those in Europe, the US, Japan, South Korea, and Australia [1,2,3]. Chloride ions penetrate concrete and reach the steel surface, where they crystallize and cause localized corrosion, which can transition to uniform corrosion, impacting the service life of concrete structures. In certain regions, calcium chloride (CaCl₂), magnesium chloride (MgCl₂), and other salts are used as road salt. When these salts dissolve in snowmelt or rainwater, they form an electrolyte solution. In such an electrolyte environment, metals undergo electrochemical corrosion. The key to extending the service life of reinforced concrete in chloride environments lies in reducing the corrosiveness of chlorides and enhancing the corrosion resistance of the steel [4].

Historically, chloride corrosion mitigation methods centered on the concrete. Rust inhibitors were used to impede chloride penetration and reduce its accumulation in the concrete, thereby improving the steel’s surface environment [5]. Although initially effective in delaying chloride diffusion and corrosion, these methods substantially increased construction loads and did not rectify inherent material flaws like cracks and voids in hardened concrete. As a result, they failed to solve the chloride diffusion issue during later service periods [6,7].

Besides delaying chloride ion diffusion, common strategies involve applying protective coatings on steel surfaces and implementing cathodic protection. For example, electrostatic spraying of thin epoxy resin layers on steel creates a dense barrier against corrosive agents [8]. Since the 1990s, epoxy-coated rebar has been widely used [9]. Another approach involves stainless steel-coated rebar, where a layer of stainless steel is applied over ordinary rebar, offering excellent corrosion resistance at relatively low production costs [10,11,12]. There are two primary methods for manufacturing stainless steel-coated rebar: one involves coating stainless steel tubing with carbon steel particles and then rolling it, while the other involves spraying a stainless steel layer onto the billet before rolling. Both methods provide a corrosion-resistant layer, but they also come with higher production costs, potential issues with uniformity and separation of the stainless steel layer, welding difficulties, and limited application scenarios due to fewer manufacturers [12,13,14,15].

Stainless steel rebar, with high Cr and Ni content, exhibits excellent corrosion resistance. However, its high cost restricts widespread application [16,17]. Therefore, the focus has shifted to developing low-cost, high-strength, and corrosion-resistant low-alloy steels [12]. In the 1990s, MMFX Steel Corporation (Las Vegas, NV, USA) introduced a type of rebar with 9.0% Cr content [18,19]. This rebar, with a martensitic—austenitic lamellar microstructure, differs from duplex (ferritic—austenitic) and conventional (pearlitic—ferritic) rebars. Its microstructure reduces electrochemical activity, enhancing corrosion resistance ~5–6-fold compared to conventional rebars, meeting marine engineering demands [20,21].

Recent studies have identified key alloying elements such as Cr [22], Mo, Ni, and Cu that improve corrosion resistance. The goal is to enhance strength while reducing alloying elements, as seen in MMFX rebar and other low-alloy steels. For example, research on Cr-Ni and Cr-Cu systems has revealed that Cr significantly improves corrosion resistance, Mo primarily aids in preventing pitting corrosion, and Cu helps in reducing rust formation and slowing corrosion [23]. Elements like phosphorus (P) and vanadium (V) further influence performance by promoting passivation and refining grain structure, respectively, [24]. Moving forward, research needs to focus on [25,26,27,28,29,30]:

• Improving strength retention of low-alloy corrosion-resistant rebars in chloride environments. Current research primarily addresses corrosion behavior, and high-strength, corrosion-resistant rebars are still in early development stages;
• Developing micro-scale and dynamic corrosion mechanisms. Existing studies focus on macro-scale observations like rust layer composition and structure, lacking detailed insights into passivation layer growth and alloying element effects;
• Establishing standardized models and prediction methods for corrosion processes and lifespan of different alloy systems. Presently, predictions are limited to weight loss rates and electrochemical characteristics, without a comprehensive theoretical framework integrating multiple factors.

Cr content in the range of 1.00–1.10% effectively lowers the self-corrosion potential and enhances passivation film formation [31]. Ni content between 0.45 and 0.50% synergizes with Cr to improve corrosion resistance and passivation film properties [16]. Mo, with 0.15–0.30%, inhibits pitting corrosion and refines grain structure, while Cu, at 0.50–0.55%, slows rust growth and reduces conductivity. P, in amounts of 0.01–0.03%, aids in passivation and corrosion reduction, though higher levels can increase brittleness. V, at 0.01–0.02%, refines grain size, enhancing strength and toughness [14,32,33].

This research innovatively devises a new low-alloy rebar with enhanced mechanical and anti-chloride-corrosion properties, distinct from traditional chloride-focused studies. It delves into the corrosion initiation and spread of high-strength corrosion-resistant rebars in chloride settings, stressing alloying elements’ role in the rust layer. By analyzing how these elements affect corrosion, the study aims to provide a scientific base for curbing corrosion in marine engineering. It integrates alloy optimization with a deep-seated look at element-rust-layer interactions, promoting a thorough grasp of corrosion behavior in high-chloride scenarios.

2. Materials and Methods

The experimental materials include 25 mm diameter 500 MPa-grade single-V low-alloy seismic-resistant rebar (designated as #1) and 500 MPa-grade Nb composite high-strength seismic-resistant rebar (designated as #2), with chemical compositions detailed in

Table 1. Chemical composition of low-alloy steel bars from the factory (wt.%).

Sample Catalog	Chemical Composition
	C	Si	Mn	S	P	Cr	Mo	Ni	Cu	V	Nb	Fe
#1	0.11	0.52	1.21	≤0.003	≤0.015	0.95	0.62	0.44	0.61	0.01	-	Bal.
#2	0.12	0.71	1.52	≤0.008	≤0.019	-	-	-	-	-	0.002	Bal.

. The #1 rebar features an alloy composition with Cr content ranging from 0.75% to 0.95% and Mo content at 0.62%, along with a V content of up to 0.010% to enhance strength. In contrast, #2 rebar does not incorporate any corrosion-resistant alloy elements. This study aims to further investigate the impact of alloying elements on the corrosion resistance of these two types of rebar. The mechanical properties of #1 rebar are as follows: yield strength ≥500 MPa, tensile strength ≥630 MPa, strength-to-yield ratio ≥1.25, post-fracture elongation ≥12%, and total elongation at maximum load ≥9%.

The preparation method for #1 corrosion-resistant rebar includes several stages: molten iron pretreatment, converter smelting, steel refining, billet continuous casting, hot rolling, and temperature control and cooling. As shown in Figure 1, After billet casting, the temperature is controlled at 1150 °C ± 50 °C, with a holding period of 30 to 50 min before hot rolling. The initial rolling temperature is set at 1140 °C ± 50 °C, intermediate rolling is conducted at 950 °C to 970 °C, and final rolling occurs at 1080 °C to 1100 °C. During rolling, no water cooling is applied, and the cooling bed temperature ranges from 840 °C to 870 °C. Post-rolling cooling is performed with a rate controlled at 2 °C/s.

The method of preparing #2 rebar includes iron pretreatment, converter smelting, steel refining, billet continuous casting, hot continuous rolling, and temperature-controlled cooling process, in which, at the end of the converter smelting, the C in the steel is 0.10~0.13% by mass percentage, and the P is ≤ 0.019%. The temperature-controlled cooling process includes: after the completion of billet continuous casting, the heating temperature is controlled at 1150 °C, holding temperature for 60 min, and then hot continuous rolling is carried out, with the opening rolling temperature at 1150~1200 °C, the intermediate rolling temperature at 980 °C, and the final rolling temperature at 1100 °C. No water penetration treatment is carried out in the rolling process, the upper cooling bed temperature is 850~900 °C, and in cooling after rolling, the cooling rate is controlled at 5 °C/s.

Microstructural analysis is performed using an optical metallographic microscope (LEICA Co., Wetzlar, Germany) to capture the microstructures of two types of steel reinforcements. The preparation of these samples involves a rigorous preprocessing procedure. Initially, an electrical discharge machine (WEN-JIE Co., Suzhou, China) is used to cut the reinforcements into 10 mm thick metallographic samples, ensuring minimal heat generation to maintain the original structure. Upon completion of sample cutting, the sample undergoes grinding and polishing procedures to guarantee its compliance with the requisite conditions for metallographic specimen collection.

The cyclic infiltration experimental tests of high-strength seismic-resistant and corrosion-resistant steel bars in the incipient stage of corrosion are conducted with the objective of investigating their corrosion initiation and propagation behaviors. The preparation of specimens for cyclic immersion tests strictly adheres to the criteria stipulated in “Testing Methods for the Corrosion of Steel Bars in Chloride-containing Environments” [34]. Specifically, this standard mandates the utilization of a 2 wt.% NaCl solution for experimental implementation. The main challenge in processing all the specimens for the cyclic immersion test of #1 Steel and #2 Steel is to achieve the required surface roughness, which must be less than or equal to 0.8. Initially, φ25 mm rebar samples are machined on a CNC lathe to produce φ10 mm smooth rods. These rods are then ground to a φ9.1 mm smooth round shape using a grinder. The specimens are subsequently cut into 50 mm long smooth rods with a wire cutting machine, and a 2.5 mm diameter hole is drilled 2 mm from one end to facilitate sample hanging. Finally, the specimens are polished using a water mill to produce the cyclic immersion test samples.

The cyclic immersion corrosion test equipment (ANAI Co., Nanjing, China) is shown in Figure 2. This equipment is designed for artificially simulating climate corrosion conditions, assessing the adaptability and reliability of products, components, and raw materials under such environmental conditions. The experimental procedure is shown in the figure below:

The box body accommodates the test chamber and multiple systems. The rotary drum control system governs sample rotation. The constant temperature and humidity system sustains the test environment’s temperature and humidity. The cooling system regulates the test chamber’s temperature. The corrosion liquid tank stores the corrosive solution for immersion tests. The electrical control system manages the overall unit operation.

The top of the box is equipped with an infrared heating system, including infrared sensors, exhaust fans, and adjustable support rods. The corrosion liquid tank is installed at the bottom of the box, allowing for the immersion and drying of samples through a lifting basket mechanism. This setup accelerates the simulation of corrosion effects on test specimens by exposing them to saltwater immersion, alternating wet and dry conditions, and infrared radiation. The equipment is easy to operate, provides accurate and reliable test data, and is suitable for corrosion testing of various metals and protective coatings. It is manufactured in accordance with “Test Methods for Cyclic Immersion Corrosion of Weathering Steel” and “Cyclic Immersion Test for Corrosion of Metal and Alloy in Salt Solutions” [35,36].

The parameters of the cyclic infiltration experiment were set as shown in

Table 2. Experimental design of periodic infiltration.

Sample Catalog	Solution	Solution Immersion Temperature/°C	Drying Temperature/°C	Relative Humidity/%	Corrosion Durations/h
#1	2 wt. %NaCl	45	70	70 ± 10	1 6 12 72 144 288 576
#2

, in which the specimens were immersed in the solution for 12 min/h and dried for 48 min/h, and the experimental durations of the corrosion budding phase were set for 1, 6, and 12 h, and the corrosion expanding phase was set for 72, 144, 288, and 576 h. The experimental parameters of the cyclic infiltration experiment are shown in Table 2. The design criteria for the cyclic infiltration experiments were divided into the pre-corrosion emergence stage and the late corrosion expansion stage, with accelerated corrosion of 1, 6, and 12 h set as the corrosion emergence stage, and 72, 144, 288, and 576 h set as the late corrosion expansion stage, and the study was carried out in two phases.

After the experiments were completed, the corrosion loss and corrosion rate were calculated for the corroded samples. The calculation formula is shown in Equation (1), where v is the corrosion rate in mm/y; w is the corrosion weight loss in g; s is the circumferential immersion surface area in mm², the specimen has a diameter of 9 mm, a cylinder length of 50 mm, and a surface area of 1570 mm²; and t is the circumferential immersion duration in h; ρ is the density of steel, take 7.8 g/m³, 24 means 24 h and 365 means 365 days.

$$v={\displaystyle \frac{\frac{w}{s·t}}{\rho }}\times {10}^6\times 24\times 365$$

(1)

The surface rust layer and the structure of the inner rust layer are analyzed by means of a scanning electron microscope (VEGA3 TESCAN, Brno, Czech Republic), and the elemental distribution of the alloy is determined by means of EDS. Scanning electron microscopy is the process of emitting an electron beam of a certain energy onto the surface of a sample and reacting with it. Based on the feedback of the various electronic signals excited on the surface of the sample, the surface morphology and the distribution of the elements and their content can be determined after the test.

SKPFM combines scanning Kelvin probe (SKP) technology with atomic force microscopy (AFM). When a conductive AFM probe nears the sample surface, a contact potential difference arises from the disparity in Fermi levels between the probe and the sample. Applying a compensation voltage nullifies the electrostatic force, and this voltage equals the contact potential difference. SKPFM measurements were performed using a Jupiter XR system (OXFORD, New York, NY, USA) in AC air mode. The scan rate was set to 2.0 Hz, the set value was 400.00 mV, and the integration gain was set to 50. The SKPM samples were mainly prepared in the following way: the samples to be tested were cut into cubic samples of 10 mm × 10 mm × 1 mm, and the surfaces to be tested were prepared in accordance with the preparation process of metal graphic samples, and the surface of the rust layer was sanded and polished by using 180-grit sandpaper to remove the rust layer that was not in close contact with the surface of the specimen matrix, and the contact surface of the specimen table was sanded and polished to a flat surface to ensure conductivity. The surface of the rust layer was polished with 180 grit sandpaper to remove the rust layer that was not in close contact with the surface of the specimen matrix, and the contact surface of the specimen table was polished and smoothed to ensure electrical conductivity, which was used to detect the morphology and potential of pitting craters and the dense rust layer on the surface of the specimen.

XRD (Rigaku Ultima IV, Rigaku, Tokyo, Japan) is used to examine the chemical composition of internal and external rust layers. XRD stands for X-ray diffraction, which is a means of obtaining information about the composition of a material and the structure or morphology of atoms or molecules within the material by analyzing its diffraction pattern through the diffraction of X-rays.

3. Results and Discussion

3.1. Microstructure

The microstructural analysis of steels #1 and #2 reveals that both exhibit a uniform distribution of pearlite–ferrite grains. The smelting process effectively controlled the carbon content, minimizing the formation of carbides in the finished rebar. Specifically, #2 steel’s microstructure displays a pearlite–ferrite matrix with a higher carbon content, as shown in Figure 3. Within this microstructure, carbides with regular shapes can be observed near the bainite grains, which negatively impacts its corrosion resistance.

Statistical data indicate that the average size of ferrite grains in #1 steel is 7.14 μm, with a maximum grain size of 26.73 μm and a minimum size of 5.87 μm. In contrast, #2 steel has an average grain size of 10.67 μm, with a maximum size of 29.53 μm and a minimum size of 2.77 μm. To enhance corrosion resistance, #1 steel has a lower carbon content. Conversely, #1 steel includes specific amounts of chromium, molybdenum, nickel, and copper as corrosion-resistant additives to mitigate its susceptibility to corrosion over its service life while maintaining structural strength.

3.2. Subsection Influence of Alloy Additions on the Corrosion Emergence Behavior of Reinforcing Bars

3.2.1. Corrosion Weight Loss and Corrosion Rate

Based on the average corrosion loss and corrosion rate presented in Figure 4 and

Table 3. Corrosion weight loss and corrosion rate in the different corrosion durations.

Corrosion Durations/h	Sample ID	Corrosion Weight Loss/g	Corrosion Rate/mm·y−1
1	#1-1	0.0068	0.004864
	#1-2	0.0043	0.003076
	#1-3	0.0073	0.005222
	#1-4	0.0052	0.003720
	#1-5	0.0058	0.004149
6	#1-6	0.0249	0.002969
	#1-7	0.0232	0.002766
	#1-8	0.0309	0.003684
	#1-9	0.0213	0.002539
	#1-10	0.0266	0.003171
12	#1-11	0.0921	0.00549
	#1-12	0.0721	0.004298
	#1-13	0.0964	0.005747
	#1-14	0.0872	0.005198
	#1-15	0.0842	0.005019
1	#2-1	0.0068	0.004864
	#2-2	0.0043	0.003076
	#2-3	0.0123	0.008799
	#2-4	0.0093	0.006653
	#2-5	0.0089	0.006366
6	#2-6	0.0349	0.004161
	#2-7	0.0432	0.005150
	#2-8	0.0377	0.004495
	#2-9	0.0421	0.005019
	#2-10	0.0422	0.005031
12	#2-11	0.1327	0.007910
	#2-12	0.1132	0.006748
	#2-13	0.1064	0.006343
	#2-14	0.1242	0.007404
	#2-15	0.1022	0.006092

, at the commencement of corrosion for each immersion duration, the corrosion loss and rate of #1 steel are lower than those of #2 steel, indicating superior corrosion resistance in #1 steel. As show in Figure 4a,b, for a 1 h immersion length, the average corrosion loss of #1 steel is 0.00588 g with a corrosion rate of 0.00421 mm/y, while #2 steel has a corrosion loss of 0.00832 g and a corrosion rate of 0.00595 mm/y, with the corrosion rate of #1 steel being 71% of that of #2 steel, showing a relatively small corrosion gap. When the immersion length is 6 h, #1 steel experiences a corrosion loss of 0.0832 g and a corrosion rate of 0.00303 mm/y while #2 steel has a corrosion weight loss of 0.04002 g and a corrosion rate of 0.00477 mm/y, with the corrosion rate of #1 steel being 63% of that of #2 steel and a discernible gap in corrosion performance starting to emerge. At a 12 h immersion length, #1 steel has a weight loss of 0.0864 g and a corrosion rate of 0.00515 mm/y while #2 steel has a corrosion weight loss of 0.11574 g and a corrosion rate of 0.0069 mm/y, with the corrosion loss of #1 steel being 74% of that of #2 steel, and at this stage both steels are transitioning from localized corrosion to uniform corrosion, and the gap in corrosion loss between the two steels gradually widens with the increase in weekly immersion length and the corrosion rate of both steels gradually decreases during the transition from local to uniform corrosion.

This phenomenon demonstrates that alloying elements such as Cr, Mo, and Ni in the steel play a crucial role in the early stage of corrosion as they inhibit the progression of the corrosion process, impede the downward infiltration of Cl⁻, and decelerate the oxidation rate of corrosion products, thereby enhancing the corrosion resistance of the steel. In summary, throughout different weekly immersion lengths, #1 steel consistently exhibits better corrosion resistance compared to #2 steel and the alloying elements in the steel effectively influence the corrosion behavior, with the corrosion rate and loss being affected by the immersion time and the transition from local to uniform corrosion.

3.2.2. Morphology of Corrosion Products in the Outer Rust Layer and Elemental Fractionation

As shown in Figure 5 for #1 and #2 corrosion early corrosion degree, that is, the cycle of immersion 1 h, 6 h, 12 h after the corrosion of the macro-appearance. Two kinds of hot-rolled ribbed steel bar substrates in the early stage of localized corrosion, in line with the characteristics of the corrosion of iron and steel material corrosion in the chlorine salt environment, emerge due to 6~12 h weekly immersion during the substrate from the localized corrosion to the transformation of the uniform corrosion. At the 1 h corrosion stage, #1 and #2 are locally corroded by the macro-appearance of the corrosion products. It can be seen that #1 local corrosion area corrosion products are mainly black-brown powdery particles, and it can be speculated that its corrosion of the initial product is mainly unstable Fe₃O₄ and a small amount of Fe₂O₃, mostly along the radial direction of enrichment in the specimen at the low end; #2 corrosion products are mainly for the yellow-brown powdery particles, the distribution of the location of the more centralized to speculate that its corrosion of the initial product oxidation of the product is most often Fe₂O₃. At the 6 h corrosion stage, #1 substrate corrosion is still localized corrosion, by the macro appearance of the substrate. The #1 matrix corrosion is still localized corrosion, and the surface corrosion expansion rate is relatively slow, displaying surface corrosion products without obvious traces of shedding; #2 in the corrosion stage has been shown, by localized corrosion, the trend of uniform corrosion transformation, and most of the areas of the matrix surface have been covered by brown corrosion products, proving that alloy control #1 in the corrosion of corrosion in the emerging stage of corrosion resistance performance is better than #2. At the 12 h corrosion stage, the #1 substrate surface has been completely covered by black powder corrosion products, the local area corrosion is more serious, oxidation is more adequate, and has produced yellow-brown corrosion products; #2 corrosion surface is mainly yellow-brown lumpy products, some of the oxidation products with poor adhesion have been detached from the corrosion surface, the outer rust layer morphology is uneven, the local location is pitted, and corrosion expansion is serious.

Figure 6 provides a comprehensive illustration of the corrosion states of Steel Bars #1 and #2 subjected to different cyclic immersion durations. This encompasses the microscopic morphologies of pitting pits and the outer rust layers of local corrosion, along with the elemental composition analysis of the material surface.

With respect to the corrosion morphologies of the steel bars, irrespective of alloying element modification, the surfaces of all steel bar specimens display corrosion to varying extents. As the corrosion time elapses, the degree of corrosion intensifies. Notably, the matrix surface of Steel Bar #2, which undergoes 12 h immersion per week, experiences the most severe corrosion. After 1 h of cyclic immersion, pitting in Steel Bar #1 occurs at a relatively slow rate, and the pitting pits exhibit a more regular shape. Alloying elements are enriched at the center and edges of the pitting pits. The enrichment of approximately 0.385% chromium and around 0.605% molybdenum predisposes the surface of the steel bar to pitting. At this stage, the initial pitting pits in Spectrum 1 assume an irregular elliptical shape, and the corrosion products at the edges form a flaky texture, potentially resulting from structural disparities during the accumulation of corrosion products. Chromium and molybdenum elements are detected at the pitting sites in Spectrum 1, with the molybdenum content amounting to 2.895%. This indicates that molybdenum confers a certain degree of resistance to the pitting corrosion under study. In the center of the pitting pits, a minor enrichment of chromium and molybdenum elements is detected. It can be inferred that in the early stage of corrosion, these enriched regions are prone to triggering pitting corrosion. The non-uniform distribution of alloying elements and the original microstructure of the matrix establish a microcell structure. Here, iron, the main component of the steel bar matrix, serves as the anode, while the molybdenum- and chromium-enriched areas function as the cathode. Due to the disparity in electrochemical activity, anodic corrosion occurs, thereby initiating pitting corrosion.

Regarding the microscopic structure and elemental composition of the material surface, the SEM images of the six regions, namely (a–f), respectively, reveal diverse microscopic features such as dispersed dark spots, larger block-shaped dark areas, and dark block-like substances of different sizes. The corresponding EDS analyses demonstrate significant variances in the types and concentrations of elements among different regions. For instance, the molybdenum content in region (a) reaches 2.895%, and the chromium content in region (c) is notably high. Additionally, the energy-dispersive X-ray spectroscopy (EDS) spectra of each region are in accordance with the elemental analysis outcomes. These findings offer a microscopic foundation for an in-depth investigation of the corrosion mechanisms, element migration, and distribution patterns of the steel bars, and are instrumental in evaluating and enhancing the corrosion resistance properties of the steel bars.

As depicted in Figure 6c and supported by the elemental analysis, the #1 substrate is predominantly exposed and remains in the local corrosion stage. The rust layer exhibits irregular cracks and bumps, yet its edges are more defined with no signs of corrosion branching or spreading. This suggests that corrosion and oxidation are confined to areas of pitting corrosion, without transitioning into uniform corrosion. The energy spectrum and elemental ratios indicate a higher surface oxygen content, signaling significant oxidation, while the lower chloride content suggests limited chloride ion enrichment on the surface. This implies that oxidation is primarily driven by iron. The corrosion product contains minimal chromium (Cr) and 0.4% nickel (Ni), which may indicate that these elements appear on the surface as the rust layer develops, supporting the late-stage densification of the corrosion product. The absence of noticeable corrosion product accumulation and low oxygen content further suggests insufficient oxidation in the pitting pits. The high Cr content (0.518%) resembles the non-uniform distribution seen in the rebar matrix after 1 h of corrosion, leading to localized corrosion. The visible corrosion appears to follow defects such as turning marks and cracks, where substrate smoothness directly impacts corrosion. After 6 h of weekly immersion, #1’s corrosion level has increased from the 1 h mark, though local corrosion remains moderate and corrosion product growth is slow. The substrate’s exposure suggests it is in a passivation or dimensional passivation stage, with surface hydroxyl oxides likely inhibiting chloride ion-induced pitting and corrosion spread. Figure 6e shows that after 12 h of cyclic immersion, the #1 surface exhibits almost no corrosion.

3.3. Influence of Alloy Additions on the Corrosion Expansion of Steel Reinforcement

3.3.1. Corrosion Weight Loss and Corrosion Rate

Based on the corrosion loss and corrosion rate data presented in Figure 7 and

Table 4. Corrosion weight loss and corrosion rate during corrosion expansion at different corrosion durations.

Corrosion Durations/h	Sample ID	Corrosion Loss/g	Corrosion Rate/mm·y−1
72	#1-16	0.3241	0.003220
	#1-17	0.2921	0.002902
	#1-18	0.3084	0.003064
	#1-19	0.3921	0.003896
	#1-20	0.3656	0.003632
144	#1-21	0.9252	0.004596
	#1-22	0.8763	0.004353
	#1-23	0.8321	0.004134
	#1-24	0.8627	0.004286
	#1-25	0.8422	0.004184
288	#1-26	1.4321	0.003557
	#1-27	1.5632	0.003883
	#1-28	1.3324	0.003309
	#1-29	1.4664	0.003642
	#1-30	1.3709	0.003405
576	#1-31	2.3651	0.002937
	#1-32	2.5694	0.003191
	#1-33	2.6812	0.003330
	#1-34	2.4468	0.003039
	#1-35	2.3741	0.002948
72	#2-16	0.6241	0.006201
	#2-17	0.6022	0.005983
	#2-18	0.6782	0.006738
	#2-19	0.6821	0.006777
	#2-20	0.5821	0.005783
144	#2-21	1.2764	0.006341
	#2-22	1.5923	0.007910
	#2-23	1.4452	0.007179
	#2-24	1.3904	0.006907
	#2-25	1.4821	0.007362
288	#2-26	2.2345	0.005550
	#2-27	2.2352	0.005552
	#2-28	2.1254	0.005279
	#2-29	2.2374	0.005557
	#2-30	2.3891	0.005934
576	#2-31	3.6341	0.004513
	#2-32	3.7621	0.004672
	#2-33	3.3362	0.004143
	#2-34	3.5862	0.004454
	#2-35	3.8213	0.004746

, it is unambiguously demonstrated that #1 steel showcases significantly superior corrosion resistance compared to #2 steel throughout all stages of the immersion regime.

At the 72 h time-point, #1 steel exhibits an average weight loss of 0.3364 g, as shown in Figure 7b with a concomitant corrosion rate of 0.00334 mm/y. In stark contrast, #2 steel experiences a weight loss of 0.6337 g and a corrosion rate of 0.00631 mm/y. As a consequence, the corrosion rate of #1 steel is merely 68% of that of #2 steel. Upon reaching 144 h of immersion, #1 steel’s weight loss registers at 0.8677 g, with a corrosion rate of 0.00431 mm/y. For #2 steel, the corresponding weight loss is 1.4372 g, and the corrosion rate is 0.00714 mm/y. Here, the corrosion rate of #1 steel is 60% of that of #2 steel. Relative to the 72 h scenario, the differential has decreased, yet it remains substantial. By the 288 h mark, #1 steel’s weight loss amounts to 1.433 g, with a corrosion rate of 0.00358 mm/y. For #2 steel, the weight loss is 2.2443 g, and the corrosion rate is 0.00557 mm/y. Thus, the corrosion loss of #1 steel constitutes 63% of that of #2 steel.

At 576 h, #1 steel manifests a weight loss of 2.4873 g and a corrosion rate of 0.00309 mm/y. For #2 steel, the weight loss is 3.6279 g, and the corrosion rate is 0.00451 mm/y. Consequently, the corrosion loss of #1 steel is 68% of that of #2 steel. At this juncture, both materials have transitioned into a homogeneous corrosion phase. Despite the fact that the disparity between them gradually diminishes over time, #1 steel persistently sustains a lower corrosion rate and weight loss.

This observation implies that although alloying elements initially inhibit the progression of corrosion, augment the stability of the rust layer, and curtail the infiltration of corrosive media, their effectiveness attenuates due to their relatively low content.

3.3.2. Surface Rust Layer Corrosion Morphology and Elemental Classification

Figure 8(a-1)–(d-2) illustrate the late-stage corrosion of #1 and #2 specimens at various immersion times: 72 h, 144 h, 288 h, and 576 h. Both types of hot-rolled ribbed steel bars enter a uniform corrosion stage as immersion time progresses, with the extent of corrosion increasing. At 72 h, both surfaces exhibit a yellow-brown oxide layer; however, #1 shows some black oxide residue and minimal rust layer peeling, while #2 has secondary oxidation with orange oxide regions and noticeable rust layer stripping. By 144 h, #1’s surface is covered with a flat, powdery yellow-brown oxide, while #2’s surface is uneven with a powdery oxide coating and expanded rust areas. At 288 h, both surfaces display significant deformation due to rust layer expansion, with #1 showing more centralized bulging and smaller sizes, whereas #2 has more dispersed bulging along the specimen’s length. After 576 h, corrosion has further intensified. Steel #1 retains its cylindrical shape with moderate bulging, while #2 suffers severe deformation with bulges reaching 2–3 mm in height, leading to significant internal rust layer thickness and corrosion product concentration. The deformation in #2 suggests that under service conditions, this could lead to structural failure of the steel matrix.

After 72 h of cyclic immersion, the surface morphologies of #1 and #2 steels, as depicted in Figure 9, present distinct characteristics. Steel #1 reveals a relatively planar rust layer, predominantly composed of a dense rust stratum and powdery corrosion by-products. In marked contrast, Steel #2 exhibits a rugged, hilly accumulation of oxides. Here, the corrosion products have gradually grown inwards, giving rise to an expanded and prominently protruding rust layer. As the immersion time progresses to 144 h, the outer rust layer of Steel #1 undergoes a transformation towards greater compactness. The powdery corrosion products on its surface have bonded and clumped together. For Steel #2, its rust layer has evolved from the initial hilly aggregation to a complex state, incorporating both dense and loose regions. Noticeable corrosion grooves demarcate these two distinct states. The loose corrosion products bear a resemblance to brown, flaky snowflakes, while the dense areas are manifested as yellowish-brown, irregular chunks. By the 288 h milestone, both Steel #1 and Steel #2 display expanded and protruding rust layers. However, Steel #1 is characterized by fewer protrusions and a more uniformly compact rust layer, with negligible structural variations. In contrast, Steel #2 showcases a greater number of protrusions, and there are conspicuous differences between its dense and granular rust components, indicating a lower degree of corrosion uniformity. At 576 h, the rust layers of both steels exhibit substantial deformation. Steel #1’s rust layer features relatively few protrusions and a uniquely gourd-shaped oxidation basin, where the protrusions and depressions are intricately and tightly interconnected. Conversely, upon examination under electron microscopy, the protrusions of Steel #2 are predominantly mushroom-shaped, with fewer depressions. Moreover, there is a greater degree of variability in the height of these protrusions and a more pronounced contrast, highlighting the non-uniformity of its corrosion process.

In summary, within a chloride-rich environment, when evaluating factors such as rust layer morphology, density, growth kinetics, and corrosion patterns, Steel #1 demonstrates a clear superiority in late-stage corrosion performance over Steel #2. This enhanced performance ultimately confers a longer service life to Steel #1.

Under the chlorine salt environment, the evolution of the outer rust layer in the late corrosion of the two bars varies greatly, #1 is better than #2 in terms of the morphology and density of the rust layer or the growth rate and growth pattern of the rust layer in the same corrosion time, which directly determines that the service time of the former is longer than that of the latter. After 72 h weekly immersion, the main alloying elements in the outer rust layer of #1 are Cr and Mo, of which Cr is enriched at the expansion, which improves the density and stability of the oxide film layer at the deformation location, and has a certain hindering effect on Cl. After 72 h weekly immersion, the corrosion products of #2 are corrugated pile up, and the top of the pile has poor density and even has been broken and peeled off, which creates conditions for Cl⁻ infiltration. After #1 was corroded by 144 h, the main component of the surface rust layer was fully oxidized Fe₂O₃, and the surface of the rust layer was relatively flat without obvious cracks and defects. Elements Cr, Mo, and Ni were enriched in the granular rust layer and the powder rust layer, among which, the area with higher content of alloying elements was prone to form a granular rust layer, and the black oxide produced played a fixing role for the granular rust layer, with a better bonding property. After 288 h corrosion, the surface of the rust layer has obvious expansion phenomenon, and there is a very high Mo content of black crystals produced on the surface of the flat dense rust layer; thus, it can be assumed that its main component is a kind of Fe-Mo compounds, with the expansion of the corrosion of the expansion of the enrichment of the external rust layer to play a role in the anchorage. At the edge of the expansion area, the rust layer is dense and there is no enrichment of V and Mo, Ni. It can be hypothesized that a certain amount of V can effectively enhance the densification of the rust layer during the expansion process and change the surface morphology of the corrosion products on the surface of the rust layer from loose and flocculent to dense and blocky. In summary, the outer rust layer completely enters the expansion stage.

3.3.3. Cross-Sectional Rust Layer Morphology

As depicted in Figure 10, during the late stage of corrosion, the rust layer structures of the two bars both exhibit a two-layer configuration, with an inner and an outer layer. The growth direction of the corrosion products aligns with the infiltration direction of chloride ions. Corrosion cracks propagate along the lateral expansion direction of the rust layer and are predominantly located in the yellowish-brown outer rust layer region.

There is a significant difference in the thickness of the rust layers between bars #1 and #2. After 72 h of corrosion, the thickness of the rust layer of #1 is 184.1 μm, while that of #2 is 453.8 μm. The thickness of #1’s rust layer accounts for 40% of #2’s. After 144 h of corrosion, the thickness of #1’s rust layer is 369.4 μm, and that of #2 is 576.3 μm, with #1’s thickness being 64% of #2’s. After 288 h of corrosion, the thickness of #1’s rust layer is 502.1 μm, and that of #2 is 620.8 μm, with #1 accounting for 80% of #2. After 576 h of corrosion, the thickness of #1’s rust layer is 732.8 μm, and that of #2 is 848.1 μm, with #1 accounting for 86% of #2. The change in the thickness ratio is consistent with the percentage of the corrosion rate at the corresponding stage.

In terms of the rust layer structure, layering is observed in both rust layers. For #1, the volume evolution of the dense area in the outer rust layer is smooth, and the outermost rust layer is flat and regular, with no obvious expansion. In contrast, the outermost layer of #2’s rust layer has an irregular shape and is uneven, showing obvious volume expansion, which is in line with its outer surface micromorphology.

In summary, throughout the late-stage corrosion process, #1 bar consistently shows a thinner rust layer compared to #2 bar, and its rust layer structure is more stable and less prone to expansion. The difference in rust layer thickness between the two bars at different corrosion times reflects their varying corrosion rates, and the distinct morphological characteristics of their rust layers further demonstrate the difference in their corrosion resistance.

3.4. Phase Composition Group of Inner and Outer Rust Layer

According to the experimental design, upon the completion of the late-stage corrosion cycle infiltration test, the corrosion products that could be collected from the sample surface were analyzed using X-ray diffraction (XRD). The aim was to determine the physical phases of the rust layer composition. Subsequently, the results were further analyzed with the aid of Jade 6.5 software. By combining the above-mentioned late-stage corrosion surface morphology and the cross-sectional structure of the rust layer, a comprehensive understanding was achieved. The corrosion products at 72 h, 144 h, 288 h, and 576 h were subjected to physical phase analysis. The scanning range for this analysis was set from 10° to 90°, with a scanning rate of 2°/s.

In the rebar matrix continuously exposed to the chloride-salt environment, Fe elements were gradually and directly oxidized to form FeOOH, FeCl₂, Fe₃O₄, and Fe₂O₃. During the long-term corrosion process, alloying elements were dissolved in the corrosion layer as the iron matrix corroded. This dissolution significantly contributed to enhancing the corrosion-resistance performance. Among these alloying elements, the Cr element played the most prominent role. The dissolution reaction of the Cr element greatly enhanced the stability and densification degree of the inner rust layer, causing the inner rust layer to exhibit an amorphous morphology and indirectly reducing the corrosion rate.

As shown in Figure 9, which presents the XRD curves of the internal and external rust layer corrosion products for each corrosion cycle of #1 and #2, it can be observed from the data curves that the physical compositions of the corrosion products of #1 and #2 are highly similar. They mainly consist of yellow-brown Fe₂O₃, black Fe₃O₄, and black γ-FeOOH and α-FeOOH homogeneous isomers. Through comparison, it was found that FeCr₂O₄ was produced in the late-stage corrosion layer of #1.

From the comparison in Figure 11a, after 72 h of corrosion, the number and intensity of the γ-FeOOH and α-FeOOH characteristic peaks in the internal and external rust layers of #1 were similar to those of #2, with no significant differences. This indicates that the percentage of the passivation phase in the rust layers was close at this time. However, the Fe₂O₃ characteristic peaks showed more contrast, and the intensity was higher at the main-peak position for #1, suggesting that the percentage of Fe₂O₃ in the corrosion product of #1 was higher, meaning a more complete oxidation.

After 144 h of corrosion, the peak intensity of α-FeOOH in the corrosion products of #1 increased, indicating an increase in the proportion of this stable phase. Moreover, the intensity of FeCr₂O₄ at the main-peak position increased significantly compared to that at 72 h. This demonstrates that at this time, the Cr element had a higher solubility in the rust layer. It replaced a portion of the Fe in the oxidation process and reacted with Fe to form spinel-structured FeCr₂O₄. The stability and binding performance of FeCr₂O₄ with other oxidation products were better than those of other iron oxides. The intensity and number of Fe₃O₄ peaks did not change much.

After 288 h of corrosion, the phase compositions of the two corrosion products showed obvious differences. The characteristic peak intensities of Fe₂O₃ and α-FeOOH in #1 reached their maximum values, indicating that the proportion of the stable phase in the corrosion products of #1 was the highest at this time. Comparing the characteristic peak numbers and intensities of the corrosion products of #2 after 144 h and 288 h of corrosion, the attenuation of the α-FeOOH peak in #2 after 288 h was particularly obvious. This indicates a higher degree of oxidation, a larger volume expansion amplitude, which is consistent with the previous results of the rust-layer structure.

When the corrosion time reached 576 h, the corrosion-product results of the two steel bars were not significantly different from those at 288 h. This indicates that after 288 h, the physical reaction of the corrosion products had entered a stable state, and the proportion of each compound was maintained within a fixed range.

In summary, during the late-stage corrosion of #1 and #2, as the corrosion time increased, the phase composition of the corrosion products eventually stabilized and no longer changed. The chemical and physical properties of the corrosion products of #1 were more stable than those of #2. This stability is attributed to the role of alloying elements, especially the Cr element, which influenced the formation and properties of the corrosion products, leading to differences in the corrosion-resistance performance between the two steels.

3.5. Morphology and Electrical Conductivity of the Dense Areas of the Rust Layer on the Outer Surface

Figure 12a–h comprehensively depict the microscopic corrosion morphology and potential distribution of the two steels at different time intervals, providing crucial insights into their corrosion behavior over time.

After 12 h of the corrosion process, as presented in Figure 12a, for steel #1, granular corrosion products are observable. These products persist even after the polishing procedure. The high-potential areas on the surface of #1 signify that the extent of pitting corrosion is minimal. This indicates that, at this early stage, the corrosion initiation on #1 is relatively well-contained, and the surface remains relatively intact in terms of localized attack. In contrast, Figure 12b shows the condition of steel #2. Its surface retains a substantial amount of corrosion products, and there are obvious pitting traces. The potential distribution along the corrosion paths reveals a significant potential difference. This potential disparity leads to the formation of a local corrosion battery. In a local corrosion battery, the areas with lower potential act as anodes, where oxidation and dissolution of the metal occur more rapidly, while the higher-potential areas function as cathodes. The presence of such a battery accelerates the corrosion process on steel #2 at this early stage.

As the corrosion time progresses to 72 h, Figure 12c,d disclose that the rust layer on both steels has mostly detached after grinding. For steel #1, it maintains a uniform negative potential, and this potential is evenly distributed across its surface. This uniform potential distribution implies that the corrosion is proceeding in a relatively homogeneous manner, without significant localization of the corrosion activity. On the other hand, for steel #2, its potential has increased from 200 mV to 400 mV. This increase in potential indicates larger local potential differences. These amplified differences further promote the development of corrosion batteries. As a result, the corrosion on steel #2 becomes more complex and intense, with a greater likelihood of accelerated localized corrosion due to the enhanced driving force provided by the more pronounced potential gradients.

At the 144-h mark, Figure 12e,f demonstrate that steel #1 exhibits a highly uniform rust layer potential. Moreover, there are notable areas of stable compounds, which are in accordance with the XRD results obtained previously. These stable compounds likely contribute to the overall stability of the rust layer and may act as a barrier to further corrosion. In contrast, steel #2 shows a distinct potential distribution. There is a high potential around the corrosion pits, while the potential in the residual rust layer is lower. This non-uniform potential distribution indicates an uneven corrosion progression. The high-potential areas around the pits suggest that these regions are more reactive, and the corrosion is likely to continue to expand and deepen in these areas, while the lower-potential residual rust layer may experience a different rate of corrosion, potentially leading to a more erratic and less-predictable corrosion pattern.

After 288 h, the corrosion products on the surfaces of both steels have largely stabilized. By 576 h, no significant potential changes are detected in either steel. However, a comparison between the two steels reveals that steel #2 displays larger and more irregular corrosion pits compared to steel #1. These larger and more irregular pits are clear indications of more severe corrosion damage on steel #2. The greater size and irregularity of the pits suggest that the corrosion on steel #2 has been more aggressive and less controlled throughout the corrosion process, resulting in a more deteriorated surface condition compared to steel #1, which has maintained a relatively more stable and less-damaged surface morphology despite undergoing corrosion.

In summary, throughout the corrosion process, steel #1 shows a more stable and uniform corrosion behavior in terms of potential distribution and surface morphology, while steel #2 experiences more complex and severe corrosion, as evidenced by the formation of local corrosion batteries, larger potential differences, and more significant pitting corrosion over time.

4. Conclusions

This comprehensive study systematically evaluates the corrosion behaviors of high-strength 500 MPa seismic-resistant steel bars (#1) and conventional carbon steel bars (#2) in chloride salt environments across both early and late stages. The key findings are synthesized as follows:

• In chloride environments, both steel types exhibit initial corrosion, yet significant disparities emerge. The low-alloy #1 steel, incorporating Cr, Mo, Ni, Cu, and V alloying elements, demonstrates reduced corrosion weight loss and lower corrosion rates compared to #2. Additions of Cr, Mo, Ni, Cu and V in contents below 1 wt.% caused improvement in corrosion resistance. Specifically, Mo enrichment at localized corrosion sites inhibits chloride ion penetration, delaying pitting corrosion development [1]. Whereas #1 steel undergoes lateral corrosion propagation along the surface, #2 steel rapidly transitions to uniform corrosion with a rust layer thickness reaching four times that of #1 steel after equivalent exposure durations;
• The corrosion products of #1 steel form a denser, more adherent rust layer with superior electrochemical stability. In contrast, #2 steel displays notable rust layer detachment and cracking after 1–6 h of immersion, with defect density increasing significantly by 12 h. These morphological defects facilitate chloride ion ingress and secondary corrosion. X-ray diffraction analysis reveals that #1 steel’s rust layer contains higher proportions of α-FeOOH and Fe₃O₄, which are more stable phases [5]. Conversely, #2 steel’s rust layer, dominated by Fe₂O₃, undergoes more pronounced expansion due to incomplete oxidation processes;
• Throughout the immersion periods (72–576 h), #1 steel consistently exhibits lower corrosion losses and reduced corrosion rates compared to #2 steel. By 72 h, the performance gap becomes statistically significant. Although the disparity narrows marginally at 144 h, it remains substantial. At the 576 h severe corrosion stage, #1 steel maintains slower corrosion expansion and lower material loss [12]. This suggests that alloying elements play a critical role in stabilizing the rust layer matrix and mitigating long-term corrosion kinetics;
• The outer rust layer of #1 steel evolves more favorably in terms of density and growth patterns. Between 72 and 288 h, Cr segregation at expansion sites enhances oxide layer stability, while Mo-rich black crystalline structures (tentatively identified as Fe-Mo compounds) reinforce the rust layer integrity through mechanical anchoring. Vanadium additions further densify the rust layer, transforming its morphology from loose flocculent aggregates to compact crystalline masses. By 576 h, #1 steel’s rust layer remains structurally intact with controlled volumetric expansion, whereas #2 steel experiences severe morphological deformation with no evidence of crack arrest at alloy-enriched zones;
• Both steels develop bilayer rust structures aligned with chloride ion infiltration pathways. However, #1 steel’s outer rust layer is characterized by greater density and limited expansion, whereas #2 steel’s outer layer is irregular and significantly swollen. While corrosion product compositions stabilize over time, #1 steel maintains superior chemical and physical stability. The consistently higher volumetric expansion rates of #2 steel at all stages reflect accelerated oxidation and material degradation.

The low-alloy #1 steel demonstrates superior chloride corrosion resistance compared to conventional #2 steel, primarily attributed to alloying elements that enhance rust layer stability and delay corrosion progression. These findings underscore the critical role of microalloying strategies in developing durable reinforcing steels for chloride-exposed infrastructure, providing a scientific basis for optimizing alloy compositions to balance mechanical performance and long-term corrosion resilience.